Converters, such as variable speed to constant frequency converters which convert a variable speed mechanical input to a constant frequency electrical output, are typically required to handle large electrical power requirements. When such converters are required to operate in high load conditions, the converters can generate a substantial amount of heat which must be removed in order to protect the components of converters from damage. Accordingly, converters are cooled to prevent such damage.
For example, it is known to operate a fan at a constant speed in order to cool a converter. When a fan is operated at a constant speed to cool a converter, the fan must be run fast enough to cool the converter during the highest expected load condition. However, a fan running at high speed frequently entrains lint and other debris which tend to clog the cooling channels of the converter over time. As lint and debris build up in these cooling channels of a converter, both the cooling efficiency of the fan system and the life expectancy of the converter decrease.
One solution to this problem is to operate the fan at a speed that is dependant upon the load on the converter. Thus, at higher loads, the fan is run at a greater speed, and at lower loads, the fan is run at a slower speed. U.S. Pat. No. 3,059,164 discloses a variable speed fan control system for cooling a rectifier in response to load. However, the fan loads down the rectifier so that the fan unnecessarily contributes to the cooling needs of the rectifier.
U.S. Pat. No. 5,291,383 discloses a fan control system in which a cooling fan is run at a variable speed depending upon load as sensed at an input to an AC/DC/AC converter. Accordingly, the speed of the fan is not controlled directly by the load as sensed at the output of the converter but is controlled indirectly by sensing the input of the converter. This fan control introduces delay between changes in load and control of the fan.
The present invention is directed to a variable speed cooling fan control which solves one or more of the above noted problems.